INTRODOCTION TO
Magnetic Resonance Imaging
MRI Efficacy
Magnetic Resonance has been called one of the most comprehensive and
efficacious diagnostic imaging modalities in medical history.1. It become a
viable clinical technique in 1982 and during its relatively short life time has
become the primary imaging modality for investigation of the brain, spinal cord ,
spine, cancellous bone, and joint. It is widely used for the identification and
staging of tumor, investigations of large blood vessels, and in pediatric studies.
Cardiac MR, with its unique ability to provide simultaneous information about
anatomy , function, and tissue character , has become a primary or complimentary
modality in a wide range of pathologies , such as aortic disease, masses,
congenital heart disease, ventricular function, and cardiomyopathies.
The Technology Behind MRI
When the human body is positioned within a
strong magnetic fields, certain nuclei – such as
proton nuclei that have inherent magnetic
properties – will align with and rotate around
the direction of the applied field. If these
nuclei are then subjected to radiofrequency
energy, they will absorb energy and move into
an excited state. When the RF energy is turned
off, the nuclei return to their equilibrium state,
releasing a detectable radiofrequency signal.
This signals is the basis for the threedimensional diagnostic images produced by
the magnetic resonance system. By varying
the specific sequence of RF “pulses” applied
to the molecular environment of the nuclei to
differentiate tissue types and detect various
dynamic processes that occur in the body.
Risks and Safety Concerns with MRI
The risk of MRI can be categorized as follows:
1. Projectile effects produced by ferro-magnetic attracted by the static
magnetic field.
2. Torsion effects on metallic clips or other devices in the patient due to
magnetic fields.
3. Failure of pacemakers and potentially of other implanted electronic devices
as a result of exposure to radiofrequency signals.
4. Heating effects, for example with tattoos or cosmetics.
Contrast Agents
Some MR exams require the use of contrast agents.
Gadolinium , an intravenous paramagnetic contrast
agent, increase the sensitivity of lesion detection,
demonstrating
metastatic
lesions
only
several
millimeters in size that are not seen on CT.
Paramagnetic contrast agents are rarely associated with
life threatening reactions and side effects are minor
compared with conventional iodinated contrast media.
MRI is ideal in diagnosis of
1. Primary and metastatic neoplasm
2. Meningiomas and extra-axial tumor
Even small meningiomas or neuromas are well demonstrated using gadolinium.
Tumors involving the foramen magnum or skull base such as chordomas are
easily seen. The relationship of tumors to major vessels and fromina is well
delineated.
3. pituitary tumors.
Micro adenomas are easily identified on direct sagital and coronal images. The
relation ship of micro adenoma to the optic chiasm , supra sellar cistern,
cavernous sinuses is well demonstrated.
Pituitary tumors are easily distinguished from para sellar mass lesion such as
carotid aneurysma or supra sellar mass lesion.
4. Ischemai
Within several hours, edema from an acute cerebral infarction is evident on
MRI; however, it is not always possible to differentiate bland from
hemorrhagic infarcts without Gadolinium contrast enhancement. CT is a
useful examination in evaluating acute stroke in the first 24 hours.
MR angiography , which is now more widely available , an depict AVM’s
and aneurysma expeditiously and non –invasively. It can also demonstrated
occlusive disease in the carotid and iliofemoral circulation.
MRI is useful in identifying small lesions involving the brain stem or
posterior fossa that may be missed on CT.
5. Adrenal Glands
MRI show promise in characterizing adrenal mass such as
pheochromocytomas. In-phase and out-of-phase images may be useful for
distinguishing non-functioning adenomas from metastatic disease.
6. Urinary bladder
MRI is useful in identifying and staging bladder carcinoma, permitting
visualization of wall invasion, perivesical extent and lymphadenopathy.
7. Uterus
MRI discriminates between endometrium, junctional zone and myometrium.
Leiomyomas are well depicted and their exact location is better
demonstrated than with ultrasound or CT.
MRI is valuable in identifying uterine and cervical carcinomas. It is also
useful for the evaluation of adnexal masses.
8. Prostate
Using an endocavitary coil, MRI useful in visualizing the prostate.
9. Biliary and pancreatic imaging
MR Cholangiography and pancreatography (MRCP) is a newly-described
technique that promises to fill the gap between ultrasound and CT imaging
and diagnostic ERCP (Endoscopic Retrograde Cholangiography and
Pancreatography). CT and ultrasound lack projectional display feature and
have limited sensitivity and specificity for biliary disease, while ERCP is a
highly invasive procedure. MRCP uses 2D and 3D TSE sequences to
provide detailed images of the ductal anatomy and allows physicians to
guage the extent and cause of the ductal pathology non-invasively.
10. Pediatric abdominak Malignancies
MRI is excellent in evaluating Wilms tumor and defining tumor extent.
Neuroblastomas are well depicted in addition to vascular invasion, lymph
node involvement or spinal encroachment.
11. Retroperitoneum
MRI is nearly equivalent to CT in detecting retroperitonial
lymphadenopathy. Also, chronic scar tissue often can be differentiated from
recurrenct disease. The sensitivity of MRI to vascular flow can be exploited
in evaluation of caval flow in patients with retroperitonial neoplasms.
Vascular abnormalities such as abdominal aortic aneurysms or inferior vena
caval thrombosis can be demonstrated.
12. Kidneys
Mass lesions are readily apparent with Gadolinium contrast, although MR
images cnnot always be used to separate benign from malignant renal
lesions. MRI is valuable in staging renal cell carcinoma, especially tumor
involvement of the renal veins and inferior vena cava. MR angiography can
be used to assess stenosis of the renal arteries.
13. Seizures
MRI provides excellent temporal lobe resolution and is the most appropriate
initial screening examination in patients with temporal lobe epilepsy and
seizure foci.
14. Orbits
Intraconal or extraconal masses are easily visualized al though small
calcifications in cavernous hemangiomas or meningiomas may not be
evident on MRI. Otic nerve enlargement is detected equally well with MRI
or CT; however extension of optic gliomas into the optic chiasm, optic tracts
or geniculate nuclei is best demonstrated with MRI. Suppressing the
dominant signal from retrobulber fat-suppression techniques available on
many MR systems provides vastly improved evaluation of intraconal lesion.
MRI is valuable in evaluating Graveis ophthalmopathy.
15. Hydrocephalus
MRI can differentiate communicating and non-communicating
hydeocephalus. Serial studies can be performed on pediatric patients without
ionizing radiation.
The dynamic aspect of CSF circulation can also be depicted in cine-mode.
16. Congenital malformations
Chari malformations, Dandy walker cysts, agenesis of the corpus callosum,
Meningoceles, arachnoid cysts and dysplasias of the cerebral cortex can be
mor comprehensively evaluated using MRI than any other single or
combination of modalities. Patients with neurofibromatosis or other
neurocutaneous syndromes may benefit from MRI.
17. ENT & Neck Mucosal neoplasms
Malignant
mucosal
tumors
of
the
nasophyarynx, oropharynx and larynx are
well demonstrated. Tissue planes are better
defined than on CT. Identification of
lymphadenopathy and vascular invasion or
thrombosis may provide more accurate tumor
staging.
MRI is very helpful in evaluating in
evaluating post-operative patients with
distorted anatomy.
Tumors in the para nasal sinuses are more
easily differentiated from obstructive
inflammation.
Thyroid and parathyroid tumors.
Even small parathyroid adenomas are generally demonstrated onMRI by
virtue of their signal characteristics. Fast imaging capabilities are important
because they reduce respiratory artifact.
Cysts and masses of the thyroid gland are easily visualized.
19. Parotid Gland.
Parotid tumors can be localized to either the deep or superficial lobe,
differentiated from lymph node mass and used to define local invasion.
20. Acoustic neuromas
Gadolinium-enhanced MRI depicts small intracanalicular tumors,
eliminating the need for air contrast CT cisternography and iodinated CT
examinations.
Cerebellopontine angle masses and their effect upon the pons or adjacent
structure are efficiently demonstrated using MRI’s multiplaner imaging
capabilities.
MR Angiography
Magnetic resonance agiography is becoming
increasingly
important.
It
is
used
most
frequently to depict a spectrum of normal to
severely stenosed and diseased blood vessels in
the body.
To
evaluate
aneurysms,
arteriovenus
malformations (AVM’s) and occlusions of the
intracranial
vessels,
and
to
screen
for
artherosclerotic disease and other arterial
occlusive disease.
21. The spine, Herniated Disc disease and degeneration
MRI is
often superior to
post
myelogram CT in demonstrating inter
vertebral idsc herniation and has the
added
advantage
of
multiplanar
presentation and degenerative disc
disease.
22. Syringomyelia
The extent of syringomyelia due to post traumatic, neoplastic or other
etiologies is well defined. MRI’s ability to image in multiple planes is most
useful for surgical planning.
Syrinx cavities are easily separated from associated solid intra medullary
neoplasms.
23. Spinal Neoplasms
The extent of intra medullary tumors such as ependymoma and astrocytoma
as well as associated syringomyelia is well depicted. Intradural extra
medullary masses such as meningiomas and neurofibromas are directly
visualized both with plain and Gadolinium –enhanced sequences.
Paramagnetic contrast enhancement is helpful in demonstrating small
intradural drop metastasis
Metastatic disease within the vertebrae and epidural tumor extension are
delineated without myelography, which may otherwise require both cervical
and lumbar puncture to detect the site or tumor blockage.
24. Congenital disorders as
Tethered cord, diastematomyelia, meningoceles and lipomas.
25. Infection as
Discitis, osteomyelitis and epidural abscess formation.
26. Joints disease.
A. Knee :MRI accurately depicts meniscal tears as well as
tears
involving
the
cruciate
and
collateral
ligaments.
MRI
demonstrates
boney
abnormalities
not
apparent on plain radiographic films.
B. Shoulder
Rotator cuff tendon tears can be detected, as well as abnormalities involving
the glenoid labrum and boney structure.
C. Other joints disease as TMJ ,
ankle and foot disease.
MRI can evaluate tendons and ligaments
around
the
osteomyelitis,
ankle,
and
osteochondritis,
other
boney
abnormalities
Avascular Necrosis
MRI is the most sensitive imaging modality for detecting ischemic necrosis
involving the hips and many other sites including the carpal and tarsal bones.
Primary Bone Tumors
MRI is more accurate than plain radiographic films or CT in staging primary
bone neoplasms.
Extension into adjacent soft tissue and joint spaces or involvement of
neurovascular bundles is delineated.
Soft Tissue Tumors
MRI provides important information regarding tumor extent and muscular,
reural, or vascular invasion.
Abdomen And Pelvis
27. Liver
Benign
lesion
such
as
cysts
and
hemangiomas are well characterized by
MRI.
28. Disorders of Myelination
MRI is the modality of choice in detecting multiple sclerosis involving the
brain or spinal cord.
29. Hemorrhage and Vascular Lesions
CT remains the modality of choice in traumatic or acute hemorrhage. Acute
hematomas less than 24hours old may have a variable MRI appearance.
30. Rectum
MRI demonstrates the extent of rectal neoplasms.
31. Breast Imaging
The primary imaging technique for examination of the breast is conventional
X-ray mammography, MR mammography is currently useful for detection
of lesions when the mammogram is not diagnostic.
32. Cardiac MR
The interest of both radiologist and cardiologist in cardiac MR has been
growing recently , Cardiac MR has become a primary or complementary
modality in a wide range of pathologies.
References
1. Partain CL, et al, Preface in : Magnetic Resonance
Imaging, Vol. 1: Clinical Principles. Philadelphia:
W.B. Saunders Company, 1988.
2. Margulis AR, Hricak H. Clinical Potential of MRI. In:
Magnetic Resonance Imaging, Vol. 1: Clinical
Principles. Philadelphia: W.B. Saunders Company,
1988, 58-69.
3. Shellock FG, Morisoli S, Kanal E. MR procedures and
Biomedical Implants, Materials, and Devices: 1993
Update. Radiology 1993; 189: 587-599.
4. Pohost GM, O’Rourke RA. Principles and Practices
of Cardiovascular Imaging. Little,Brown Company,
1991.
ASS.PROF,DR,AL ZABEDI A.K.
CONSULTANT RADIOLOGIST
SANA,A UNIVERSITY